1. Field of the Invention
The present invention relates to an injection molding method with compression and an apparatus therefor, and more particularly to an injection molding method and apparatus with compression suitable for molding a product such as, for example, an optical lens which requires high molding accuracy and uniformity in the compression thereof.
2. Description of the Prior Art
In order to obtain a precise molded product by using thermoplastic resin, the injection molding method with compression has lately attracted attention.
Thermoplastic resin such as, for example, PMMA (methacrylic resin) increases its dynamic rigidity to harden by cooling the thermoplastic resin from the melted state having a high temperature, and the volume of the resin is reduced with a decrease of the temperature.
More particularly, the thermoplastic resin, when cooled, hardens to from a solid product, while only simple cooling and hardening causes failures in the molded product such as shrinkage and warpage due to a decrease of the volume in the solid state thereof as compared with that in the melted state thereof.
Accordingly, a compression margin corresponding to the contraction rate in the molding is generally provided in the joint surface of the mold. Then, after injection of the melted thermoplastic resin into the mold, the thermoplastic resin is cooled to harden in the applied state with a mold clamping force.
Various process control methods for the injection molding with compression have been proposed heretofore. Basically, the temperature of the mold is previously set in the vicinity of the extraction temperature in order to increase the cooling efficiency and the mold is pressurized immediately after completion of the injection so that the capacity in the mold is equal to the volume of the molded product under the normal temperature and pressure. Then, the pressurized force is controlled to be gradually reduced with the contraction due to the cooling.
Namely, in the conventional process control, in order to improve the shape and dimensions effected by the contraction due to cooling, the pressure applied to the mold is controlled so that the thermoplastic resin to be molded maintains the volume under room temperature and atmospheric pressure in the whole range of temperature in the cooling. Thus, a satisfactory molded product can be obtained with stability of the shape and dimension.
The dynamic rigidity of the thermoplastic resin does not increase uniformly from the melted state thereof of a high temperature to the solidified state of room temperature and suddenly increases and hardens from a certain temperature (the glass transition point Tg).
Accordingly, if any deviation in the temperature occurs in each portion of the thermoplastic resin upon exceeding the glass transition point in the cooling, partially solidified portions and partially melted portions are mixedly produced in the thermoplastic resin in the molding space due to the deviation in the temperature. If the solidified portions and the melted portions are continuously pressurized uniformly, the solidified portions are apt to be subjected to plastic deformation and the inner composition of the molded product is liable to lack uniformity.
It is a matter of course that if the temperature of the mold is previously set to a high temperature and the mold is cooled for a sufficient time, since the temperature deviation in each portion of the resin is reduced, the above problems are alleviated to a certain extent. In the case of the molded product having a large size in the dimension or a partially large difference in thickness, if the resin in the mold is cooled slowly so that the temperature deviation does not occur, the operation efficiency is greatly deteriorated.